How will the pandemic end? The science of past outbreaks offers clues.

The answer depends on many factors, perhaps the most critical being the global nature of the crisis.

After months of encouraging trendlines, July’s dramatic spike in global COVID-19 infections has dimmed the proverbial light at the end of the pandemic tunnel.

In May, coronavirus cases were declining across the U.S., parts of Europe, and the Middle East as vaccination rates rose, spurring an easing of social and travel restrictions and a wave of business reopenings. But in the U.S., at least, any celebration was short-lived. By July, vaccination rates flatlined and highly transmissible coronavirus variants swept the nation, forcing health officials to reimpose masking recommendations and call for increased inoculations.

The World Health Organization declared COVID-19 a pandemic on March 11, 2020. After 17 grueling and chaotic months, weary people are wondering: When will the pandemic finally end?

“Even among the scientific community, you would get really different answers,” says Rachael Piltch-Loeb, a researcher and fellow with the Emergency Preparedness Research, Evaluation & Practice Program at the Harvard T.H. Chan School of Public Health. “There is no one definition of what the end of a pandemic means.”

A pandemic is by definition a global crisis. Lifting some U.S. public health measures and interventions “gave people a sense that the panic was waning,” Piltch-Loeb says. That euphoria blinded many to the worldwide reality, which remains bleak.

“Until this [virus] is controlled or more limited globally, it’s not going away,” Piltch-Loeb says. That means declaring the pandemic’s “end” may be a distant goal, requiring different conditions depending on who’s asked.

Where do diseases go?

When the worldwide spread of a disease is brought under control in a localized area, it’s no longer a pandemic but an epidemic, according to the WHO. If COVID-19 persists globally at what the WHO judges to be “expected or normal levels,” the organization will then re-designate the disease “endemic.”

At that stage, SARS-CoV-2 will become a circulating virus that’s “less consequential as we build immunity,” says Saad Omer, an epidemiologist and director of the Yale Institute for Global Health. (Read more about how we’ll live with COVID-19 as an endemic disease.)

Only two diseases in recorded history that affect humans or other animals have ever been eradicated: smallpox, a life-threatening disease for people that covers bodies in painful blisters, and rinderpest, a viral malady that infected and killed cattle. In both instances, intensive global vaccination campaigns brought new infections to a halt. The last confirmed case of rinderpest was detected in Kenya in 2001, while the last known smallpox case occurred in the U.K. in 1978.

Joshua Epstein, professor of epidemiology in the New York University School of Global Public Health and founding director of its Agent-Based Modeling Laboratory, argues that eradication is so rare that the word should be wiped from our disease vocabulary. Diseases “retreat to their animal reservoirs, or they mutate at low levels,” he says. “But they don’t typically literally disappear from the global biome.”

Most causes of past pandemics are still with us today. More than 3,000 people caught the bacteria that cause both bubonic and pneumonic plague between 2010 and 2015, according to the WHO. And the virus behind the 1918 flu pandemic that ravaged the globe, killing at least 50 million people, ultimately morphed into less lethal variants, with its descendants becoming strains of the seasonal flu.

As with the 1918 flu, it’s likely the SARS-CoV-2 virus will continue to mutate, and the human immune system would eventually adapt to fend it off without shots—but not before many people fell ill and died. “Developing immunity the hard way is not a solution that we should be aspiring to,” Omer says.

Finding ways to slow the spread of a disease and manage its effects is by far the safer path, experts say. Today, for instance, pest control and advanced hygiene keep the plague at bay, while any new cases can be treated with antibiotics.

For other diseases, such as the flu, vaccines can also make a difference. The available COVID-19 vaccines are highly safe and effective, which means getting enough people vaccinated can end this pandemic faster and with lower mortality than natural infections alone.

Why we need vaccines for all

WHO Director Tedros Adhanom Ghebreyesus last week reinstated a goal of vaccinating at least 10 percent of every nation’s population by September, with the loftier goal of reaching 40 percent global inoculation by year’s end and 70 percent by mid-2022.

To date, though, just 28 percent of the world’s population has received at least one dose of a COVID-19 vaccine. And vaccine distribution remains wildly lopsided. The European Union has nearly three-fourths of its eligible population at least partially inoculated; the U.S. has vaccinated 68 percent of people 12 and older.

But other nations that have lost many people to COVID-19—including Indonesia, India, and many of the countries in Africa—are working at a much slower pace. That’s in part because Covax, the United-Nations-backed program to vaccinate the world, has struggled to acquire and deliver vaccines for the world’s poorest countries. This week, the WHO issued a plea for wealthy countries to donate vaccine doses to poorer nations before offering booster shots to their own populations.

Even in countries with sufficient supply, the pace of vaccinations is influenced by hesitancy and misinformation. In the U.S., the daily rate of new vaccinations has plateaued, slowing to an average of 615,000 injections a day, or an 82 percent decline from the peak of vaccine uptake on April 13. U.S. hospitals are filling up with patients as caseloads increase in unvaccinated areas.

With more opportunities to spread and mutate, the virus has developed new variants that are not only more contagious, but more evasive. Delta is the most contagious one detected so far. The variant was first detected in India, where it helped drive one of the world’s worst surges in April. More recently, Delta contributed to a dramatic outbreak in Indonesia; antibody data suggest that more than half the population in the capital city of Jakarta have been infected. Initial research also shows the Lambda variant may be resistant to some vaccines.

The complexity of fighting a rapidly mutating virus “means that sometimes we take two steps forward and one step back,” says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

Who gets to say it’s over?

There is another option, scientists and historians say: People will decide the pandemic is over, long before any governing body declares it so.

It’s happened in the past: The 1918 flu hit in the throes of World War I, and as the fighting ended, there was a “feeling of wanting to put that whole decade to bed, and to embrace a new future,” says Naomi Rogers, professor of the history of medicine and of history at Yale University. The public entered the “Roaring Twenties” despite the flu virus still circulating throughout the U.S. population.

If society attempts to declare an end to the pandemic before science does, we’d be accepting its severe outcomes—including death. That’s often been the case with past pandemics. The flu is no longer considered a pandemic and is now an endemic disease; between 12,000 and 61,000 people in the U.S. still die from the flu each year, based on CDC estimates.

“If we can bring the death count down to a certain level and resume our lives normally, one could say the pandemic has ‘ended’,” says Jagpreet Chhatwal, a decision scientist at the Massachusetts General Hospital Institute for Technology Assessment in Boston. Again, vaccines make a difference. COVID-19 deaths in the U.S. have been tamped down in highly-vaccinated areas.

Nationally, the Centers for Disease Control and Prevention will likely provide guidance on when the pandemic has reached endemic status in the U.S., says Piltch-Loeb. That in turn will offer a path back to some version of normal life, regardless of global declarations.

“We want to go back to what it was like before COVID,” says Andrew Azman, an epidemiologist at Johns Hopkins Bloomberg School of Public Health. “It’s not going to take the WHO saying the pandemic is over for people to do that.”

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